The present invention relates generally to a projection optical system, a projection exposure apparatus having the projection optical system, and a device fabrication method, and more particularly to a catadioptric projection optical system that uses a mirror for a projection optical system that projects and exposes a reticle pattern onto a wafer.
The photolithography process for fabricating semiconductor integrated circuits employs a projection exposure apparatus that uses a projection optical system to project and expose a pattern on a mask or reticle onto a wafer to which photoresist and the like are applied. The recent, more highly integrated circuits require stricter specification and performance for a projection exposure optical system.
The projection exposure optical system needs a shorter exposure wavelength and/or a higher numerical aperture (“NA”) to improve resolution. With a short exposure wavelength ranging such as 193 nm (ArF), 157 nm (F2) and the like for higher resolution, transmission optical elements other than quartz or calcium fluoride lenses hardly provide high transmittance. Therefore, lens materials are limited to quartz and calcium fluoride for an expected light intensity. An optical system for a projection exposure apparatus using light in such a wave range as 193 nm and 157 nm includes only dioptric lenses as in Japanese Patent Application, Publication No. 10-79345 (corresponding to EP A1 828172). An optical system having many lenses made of a glass material with a large total thickness absorbs the large amount of light, and reduces the exposure dose on a wafer, causing a decrease in throughput. The lens's heat absorption and resultant temperature rise disadvantageously fluctuate a focal position, (heat) aberrations, etc. While the quartz and calcium fluoride lenses are viable to an exposure wavelength of 193 nm, small differences in their dispersion values have difficulties in corrections to chromatic aberrations and the correction to chromatic aberrations needs plural achromatic lenses with an achromatic surface and a small radius of curvature. Plural achromatic lenses in the optical system will increase the total thickness of the glass materials, which enhances the problems of lowered transmittance and occurrence of heat aberration. Calcium fluoride can hardly provide a lens with designed performance suitable for a projection optical system, and complicates a fabrication of a large-aperture lens. This complicates color corrections, and increases the cost. For an exposure wavelength of 157 nm, only calcium fluoride is usable for materials for a lens and only a single material has difficulties in chromatic aberration corrections. Since it is hard to construct a projection optical system just by using a dioptric system, various proposals that use a mirror for an optical system have been made to solve the disadvantageous reduced transmittance and difficult chromatic aberration corrections.
For example, a catoptric projection optical system including only mirrors is disclosed in Japanese Patent Application Publication No. 09-211332 (corresponding to U.S. Pat. No. 5,815,310), 10-90602 (corresponding to U.S. Pat. No. 5,686,728), etc. A catadioptric projection optical system combining a mirror and a lens is disclosed in U.S. Pat. No. 5,650,877, Japanese Patent Applications, Publication Nos. 62-210415, 62-258414, 02-66510 (corresponding to U.S. Pat. No. 4,953,960), 03-282527 (corresponding to U.S. Pat. No. 5,220,454), 05-188298 (corresponding to U.S. Pat. No. 5,668,673), 06-230287 (corresponding to U.S. Pat. No. 5,592,329), 10-3039 (corresponding to EP A2 816892), 2000-47114 (corresponding to EP A2 989434), 08-62502 (corresponding to U.S. Pat. No. 5,861,997), and 2002-83766 (corresponding to EP A2 1168028) etc.
In configuring a projection optical system that includes a reflective optical system with a shorter exposure wavelength and a higher NA and, it is desirable, in addition to feasible chromatic aberration corrections, to maintain a large enough imaging area ideally on an image surface, secure a sufficient working distance image-side, and provide a simple structure. If a large enough imaging area is obtainable on an image surface, a scanning type projection exposure apparatus will be advantageous in terms of throughput, thus, making it possible to control exposure fluctuations. If a sufficient image-side working distance can be secured, that is desirable from the viewpoint of constructing an apparatus's auto-focusing system, a wafer-stage's transport system, and the like, a simple structure would not complicate a mechanical lens-barrel and the like, thus being a merit to assembly production.
When prior art examples are thus viewed, U.S. Pat. No. 5,650,877 arranges a Mangin mirror and a refractor in an optical system, and exposes a reticle image onto a wafer. Disadvantageously, this optical system blocks light on a pupil's central part for all the angles of view to be used (hollow illumination), and cannot enlarge an exposure area. An attempt to enlarge the exposure area results in the undesirable expansion of the light blockage on the pupil's central part. In addition, since a refractive surface of the Mangin mirror forms the light splitting surface that halves light intensity when the light passes through its surface, and reduces light intensity down to about 10%. Japanese Patent Applications, Publication Nos. 09-211332 and 10-90602 basically use a catoptric optical system, but have difficulties in securing a sufficient width for the imaging area on the image surface, because of problems, such as deteriorated aberrations (the sum of the Petzval terms) and complicate mirror arrangements. Even an apparatus that includes a concave mirror with a strong power mainly near the image surface as an imaging function can hardly provide a high NA. A convex mirror arranged at a position right just before the concave mirror does not provide a sufficient image-side working distance. Japanese Patent Application Publication Nos. S62-210415 and S62-258414 apply Cassegrain and Schwarzschild mirror systems, and propose an optical system that has an opening at the center of the mirror for a hollow illumination to the pupil to image only the pupil's periphery. However, there is a concern about the influence of the hollow illumination to the pupil on the imaging performance. An attempt to lessen the hollow illumination to the pupil inevitably adds to the power of the mirror and enlarges a reflection angle incident upon the mirror. An attempt to have a higher NA causes a mirror's diameter to grow remarkably. According to Japanese Patent Applications, Publication Nos. 05-188298 and 06-230287, the deflected optical path complicates an apparatus's configuration. A high NA is structurally difficult because the concave mirror is responsible for most powers in the optical elements for imaging an intermediate image onto a final image. Since a lens system located between the concave mirror and the image surface is a reduction system and the magnification has a positive sign, the image-side working distance cannot be sufficiently secured. Since an optical path needs to be split, it is structurally difficult to secure an imaging area width. The insufficient imaging area width is not suitable for foot-printing in a large optical system.
Japanese Patent Applications, Publication Nos. 02-66510 and 03-282527 first split an optical path using by the light-splitter, and complicate the structure of a lens-barrel. They need the light-splitter with a large diameter and if the light-splitter is a prism type, a loss of the light intensity is large due to its thickness. A higher NA needs a larger diameter and increases a loss of the light intensity. Use of a flat-plate beam splitter is also problematic even with axial light, because it causes astigmatism and coma. In addition, asymmetrical astigmatism due to heat absorptions and aberrations due to characteristic changes on the light splitting surface complicate accurate productions of the light splitter.
Japanese Patent Applications, Publication Nos. 10-3039 and 200047114 propose a twice-imaging catadioptric optical system for forming an intermediate image once. It includes a first imaging optical system that has a reciprocating optical system which includes concave mirrors to form an intermediate image of an object (e.g., a reticle), and a second imaging optical system that forms the intermediate image onto a surface of a second object (e.g., a wafer). Japanese Patent Application, Publication No. 10-3039 arranges a first plane mirror near the intermediate image for deflecting an optical axis and light near the intermediate image. The deflected optical axis is made approximately parallel to a reticle stage and is deflected once again by a second plane mirror, or an image is formed onto a second object without a second plane mirror. In Japanese Patent Application, Publication No. 2000-47114, a positive lens refracts light from a first object (e.g., a reticle), and a first plane mirror deflects the optical axis. A second plane mirror in a first imaging optical system again deflects the light reflected by a reciprocating optical system that includes a concave mirror to form an intermediate image. The intermediate image is projected onto a second object (e.g., a wafer) with a second imaging optical system. Thus, both references inevitably arrange the first object surface (e.g., a reticle), a lens, plane mirror and the deflected beam close to one another, and create a problem of interference between the first object surface (e.g., a reticle) or a reticle stage and a lens or a plane mirror or an insufficient space.
Optical systems in FIGS. 13 and 9 in Japanese Patent Application, Publication No. 2002-183766, and an optical system in FIGS. 7 and 9 in Japanese Patent Application, Publication No. 08-62502 are a three-time imaging catadioptric optical system for forming an intermediate image twice. It includes a first imaging optical system for forming a first intermediate image of a first object (e.g., a reticle), a second imaging optical system that includes a concave mirror and forms a second intermediate image from the first intermediate image, and a third imaging optical system for forming the second intermediate image onto a third object surface (e.g., a wafer). The second imaging optical system includes concave mirrors as a reciprocating optical system. The optical system with an NA of 0.75 in FIG. 13 of Japanese Patent Application, Publication No. 2002-83766 arranges a plane mirror (reflection block) near the first and second intermediate images, and aligns optical axes of the first and third imaging optical systems with each other. Thus, the first object (e.g., a reticle) and the second object (e.g., a wafer) are arranged in parallel. However, a higher NA disadvantageously makes an overall length (or a distance from the first object to the second object) too long to correct aberrations. The plane mirrors (reflection block) necessary to deflect light near the positions of the first and the second intermediate image cause dust and flaws to greatly affect the imaging performance of the two plane mirrors performance. Since the first imaging optical system maintains a large reduction magnification (corresponding to a paraxial magnification |β1| of about 0.625 of the first imaging optical system), the first intermediate image needs to increase a NA by the reduction magnification against an object-side NA at the first object (e.g., a reticle), thus increasing an incident angle range upon the plane mirror. As the NA becomes higher, this problem becomes more serious: The first imaging optical system that is too responsible for a reduction magnification with a higher NA excessively increases the incident angle range upon the plane mirror, and a coating on the plane mirror causes a large difference in reflected light's intensity between p-polarized light and s-polarized light. In addition, the first imaging optical system that is too responsible for the reduction magnification lowers an image point of the first intermediate image, and makes it difficult for the plane mirror to reflect all the light at the lowest view angle onto the second imaging optical system. The optical system with NAs of 0.45 to 0.5 in FIGS. 7 and 9 in the latter Japanese Patent Application, Publication No. 08-62502 is similarly a catadioptric projection optical system for forming an image three times or an intermediate image twice. This type of a projection optical system needs another plane mirror to arrange a first object (e.g., a reticle) and a second object (e.g., a wafer) in parallel. In that case, as described in the above references, a mirror is arranged in the first imaging optical system, and provides the same arrangement as the optical system in FIG. 13 of Japanese Patent Application, Publication No. 2002-83766, if arranged near the first intermediate image. The reduction magnifications in the first and second imaging optical systems significantly affect the system's reduction magnification (where the first imaging optical system has a paraxial magnification of |β1| of about 0.438 to 0.474), and an attempt at a higher NA poses a fatal problem similar to the optical system in the former Japanese Patent Application, Publication No. 2002-83766.